When an optical fiber is drawn from silica glass in an optical fiber manufacturing process, the circumference of the freshly drawn optical fiber is immediately coated with a coating resin. Mainly, ultraviolet (UV) coating resin is used as the coating resin for an optical fiber. Urethane acrylate series or epoxy acrylate series are used for the UV coating resin.
An optical fiber increases its transmission loss due to various stresses and/or micro-bends caused by those stresses. To protect an optical fiber from such stresses, a conventional optical fiber is coated with two layers of coatings; a soft inner layer and a hard outer layer. For the inner layer, which contacts with a silica glass, a relatively lower Young's modules coating is used as a buffer layer (called primary layer hereafter). For the outer layer, a relatively higher Young's modules coating is used as a protective layer (called secondary layer hereafter). For a conventional optical fiber, a resin with Young's modules below 3 MPa is used for the primary layer and a resin with Young's module above 500 MPa is used for the secondary layer.
In general, optical fiber drawing process is performed by heating a perform, which is mainly made of silica, by a furnace, then a drawn fused silica fiber is coated with liquid UV coating resin using a coating die and exposing the coated fiber to UV light to harden the UV coating resin to form the primary and the secondary layers. The structure of the optical fiber is shown in FIG. 1.
Furthermore, in the next process, a colored optical fiber is made by coating an additional coating layer such as a colored resin onto the optical fiber.
When such optical fiber is placed under water, sometimes transmission loss increases. There have been various suggestions to create a highly reliable optical fiber by reducing the transmission loss increase even when the fiber is placed under water for a long period of time such as improving adhesive force between the primary layer and a glass optical fiber.